The present invention relates to a transflective liquid crystal display device (LCD) and, more particularly, to an IPS transflective LCD.
Transflective LCDs having subpixels each including transmissive and reflective portions are used as display units for mobile devices.
In such a transflective LCD, a liquid crystal material is sandwiched between a pair of plates or substrates. An electric field is applied in a direction perpendicular to the plane of the substrates to drive the liquid crystal material. This is known as the vertical field type. Furthermore, steps are formed on the transmissive and reflective portions to match the transmissive and reflective portions with respect to characteristics. In addition, retardation film is formed between the polarizer and the liquid crystal layer.
An IPS liquid crystal display device is one of known liquid crystal display devices. In such an IPS LCD, pixel electrodes (PIX) and a counter electrode (CT) are formed on the same substrate. An electric field is produced by the pixel electrodes and the counter electrode. Brightness/darkness levels are controlled by rotating the liquid crystal material within the plane of the substrates. Therefore, the IPS LCD has the feature that when the viewing screen is viewed obliquely, the shades of gray of the displayed image are not inverted.
In an attempt to make effective use of this feature, fabricating a transflective LCD using the IPS LCD has been proposed, for example, in patent reference 1 below.
However, where a transflective liquid crystal display is built using the IPS LCD, there is the following problem. If a retardation film is not used, and if each transmissive portion operates in normally black mode, for example, each reflective portion operates in normally white mode. Consequently, bright and dark parts of image are inverted between the transmissive and reflective portions.
Accordingly, in an attempt to solve the foregoing problem, the present applicant has filed a patent application for a transflective liquid crystal display device having a novel pixel structure (see patent reference 2 below).
In this transflective LCD already filed for a patent, the transmissive portions operate in normally black mode. That is, when no voltage is applied, the transmissive portions display black. The reflective portions operate in normally white mode. That is, when no voltage is applied, the reflective portions display white.
In this transflective LCD already filed for a patent, the pixel structure of each subpixel is so designed that the transmissive and reflective portions use respective independent counter electrodes but use a common pixel electrode. Different reference voltages (counter voltages or common voltages) are applied to the counter electrodes, thus preventing inversion of bright and dark parts of image between the transmissive and reflective portions.
Prior art literature associated with the present invention includes the following references.
Patent reference 1 described below states, “light passes through the liquid crystal layer twice in the region A that is a light reflective part. On the other hand, light passes through the region B once, the region B being a light transmissive portion. This produces a difference in optical transmissivity. To avoid problems due to the transmissivity difference, the region B is operated in first-order birefringent mode. The region A is operated in second-order birefringent mode.”Furthermore, the reference states, “display on each of the regions can be optimized by making the liquid crystal layer in the region A using the second-order birefringent mode about three times as thick as, i.e., from about 2.5 times to 3.5 times, the liquid crystal layer in the region B using the first-order birefringent mode.”
[Patent reference 1] JP-A-2003-207795
[Patent reference 2] Japanese Patent Application No. 2005-322049